In the continuous casting of steel, molten steel is delivered to a mold by means of a refractory tube which is submerged in the liquid steel. This refractory tube is referred to as a submerged entry nozzle and, in the case of slab casters, includes a central bore that terminates into two exit ports that extend transverse to the central bore. The purpose of the submerged entry nozzle is to prevent reoxidation of the steel. Aluminum is added to the molten steel to remove oxygen. While this may reduce or eliminate oxygen, it also has the undesirable side-effect of possibly clogging the passages of the nozzle with accretions of aluminum oxide. In conventional casting methods, nitrogen gas, argon gas or a mixture of the two gases is injected into the nozzle during casting to scrub the build up of accretions of aluminum oxide on the inside of the passages and to prevent non-metallic inclusions from adhering to the inside of the nozzle.
In the mold, a liquid slag layer is formed on the steel meniscus by adding or distributing mold powder into the mold on top of the molten steel. This liquid slag layer acts as both a lubricant in that it flows into the gaps between the solidifying steel shell and the mold as the molten steel solidifies, and as an insulator in that it inhibits heat from escaping the meniscus of the liquid steel.
To ensure an adequately thick slag layer, and thereby prevent the freezing of the steel near the meniscus, the temperature of the steel near the meniscus must be maintained sufficiently high. This is attained in conventional casting by the injection of argon gas into the submerged entry nozzle. The argon gas affects buoyancy in the liquid steel so that as the steel exits the exit ports of the nozzle it tends to rise towards the meniscus and therefore maintain a temperature sufficient to withstand freezing.
A deficiency in the production of molten steel and, in particular, ultra low carbon (ULC) and low carbon steel for exposed automotive applications, is the so-called pencil pipe defect. Pencil pipe defects arise from the entrapment of agglomerates of non-metallic inclusions and bubbles of argon gas under the solidifying shell of the steel being cast. The steel emerges from the caster in the form of a slab which is rolled down to a thin strip and collected as a coil. During subsequent processing of the strip the gas bubbles trapped under the skin of the strip, but now much closer to its surface, expand and form a blister on the surface of the finished product. Therefore, while use of argon gas reduces clogging, improves the slag layer thickness and increases the temperature near the meniscus, it also causes the undesirable pencil pipe defect due to trapped agglomerates of gas bubbles and inclusions.
The number of pencil pipe defects can be eliminated or substantially reduced by eliminating the injection of argon gas into the nozzle. However, in the absence of argon gas injection, it has been found in practice that there is a reduction in the slag layer thickness and, consequently, an increased risk that the steel near the meniscus will freeze. This can lead to the formation of surface defects known as "slivers".
These undesirable side-effects can be avoided, or their occurrence substantially reduced, by appropriately modifying the structure of the submerged entry nozzle, which is the object of the present invention.